For a Single Layer
Click the S or the U for that layer.
For One or More Layers
Highlight the layer(s) and
Press the <Alt> key as well as S or U to make the highlighted layer(s) selectable or unselectable, respectively.
Or
Use the Select or Unselect buttons at the top of the dialog box.
For ALL Layers
Click the Swap Sel button to toggle the selectability of all layers.